TACTILE SENSATION

Question 1

Describe the tactile sensation pathway from periphery to CNS.

Answer 1

touches reaches CNS via dorsal root afferent neurons

Question 2

What are primary afferent neurons?
What are the 2 branches?

Answer 2

cell bodies in dorsal root ganglia (body) or cranial ganglia (head)

• peripheral axon branch: to the skin (where it has specialized mechanosensory endings)
• central axon branch: to the spinal cord and brainstem

Question 3

How might touch afferent neurons encode different somatosensory stimuli (poke feels different than pinch or itch)?

Answer 3

different receptive fields for different neurons
- SA vs. RA

different receptor types for different touch sensations
- some receptors are specifically tuned for vibration, light touch, etc.
- low vs. high threshold

Question 4

What force-transducing molecules can sense mechanical force?

Answer 4

all have mechanically-gated ions – can turn force into energy very quickly

• ENaC family (important in worms, some evidence for roles in flies and mammals)
• TREK1 (K+ channel, not likely very important in touch)
• TRP family (part of mechanosensitive channels in flies and worms, no direct role discovered yet in mammals)
• Piezo family (large protein with many transmembrane segments, evolutionarily conserved across animals)

Question 5

What is dermatome?

Answer 5

area of the body sending inputs to a specific segment of the spinal cord

Question 6

Is the body necessary for the feeling of being touched?

Answer 6

no – feeling of touch on the body is ultimately generated in the brain (and that activity is sufficient to generate the percept)

Question 7

How is the representation of a particular sense in the cortex often organized?

Answer 7

according to some continuous parameter of the stimuli

Question 8

What do tactile circuits help preserve?

Answer 8

spatial information, as it travels from periphery to cortex

Question 9

Is touch detected by multiple receptor types?

Answer 9

yes – detected by a variety of receptor types, which each respond to a particular type of touch

Question 10

What do different receptors produce?

Answer 10

different types of afferent responses (ie. slowly adapting vs. rapidly adapting)

Question 11

Why is it useful to have differing spatial acuity between body areas (not just have high spatial acuity everywhere)?

Answer 11

–

Question 12

Describe the tactile specialization of etruscan shrews.

Answer 12

• accurately identify prey based only on tactile information from whiskers – have no eyes
• can tell prey is a cricket, and can also find exactly where to bite it (just below head) for maximum impact

Question 13

Describe the tactile specialization of rodent whiskers.

Answer 13

• somatotopic organization principle – anatomy of primary somatosensory cortex that encodes the whiskers has great layout
• each barrel has neurons that encode different properties of that whisker – direction, position, texture, etc.

Question 14

Describe the tactile specialization of star-nosed moles.

Answer 14

• completely blind, but has the highest concentration of tactile receptors and the fastest tactile identification behaviour ever observed
• have extremely high density of touch receptors on the rays of their stars – use rays to actively probe the environment to find food
• rays (nose) are over-represented in S1

Question 15

What distinguishes the perception of a gentle poke from a hard pinch?

Answer 15

from a peripheral standpoint:
- different receptor types that encode different things (ie. light touch, pain), each with different thresholds

from a ‘coding in the brain’ standpoint:
- differences in the pathways that transmit the response from periphery to brain

Question 16

Temperature detection of thermoreceptors vs. nociceptors.

Answer 16

thermoreceptors have lower detection thresholds than nociceptors

Question 17

Paper 1 Epilogue: What did the same researchers discover about rats and tickling in a different paper?

Answer 17

they used tickling to train rats to play hide and seek and determined that rats seek out tickling, but will disengage if there’s too much tickling

Question 18

Paper 1 Epilogue: Self-tickling

Answer 18

allo-touch and tickling both elicit USVs and trunk S1 firing
- self-touch suppresses USVs and trunk S1 firing
- this effect is mediated by GABA
- self-touch suppressed USVs even when elicited by electrical stimulation of layer 5 trunk somatosensory cortex

rats were trained to nose-poke a button for a tickle reward
- after training, rats would poke the button and then freeze (fear response)
- sometimes they would also emit alarm calls
- such ambivalence (“Nervenkitzel”) resembles tickle behaviours in children

Question 19

What is a Merkel cell?

Answer 19

very close to skin surface, good at tactile sensation (ie. braille)

Question 20

What is a raster plot?

Answer 20

shows action potentials over time

Question 21

What is the structure of the Piezo family?

Answer 21

Piezo family genes encode channel subunits with many transmembrane domains

Question 22

What does area 1 of S1 encode?

Answer 22

mostly touch

Question 23

What does area 2 of S1 encode?

Answer 23

both touch and proprioception

Question 24

What does area 3a of S1 encode?

Answer 24

mostly proprioception

Question 25

What does area 3b of S1 encode?

Answer 25

mostly touch

Question 26

What is needed for effective object localization/recognition?

Answer 26

need to both touch object, and know where arm is (proprioception) for

Question 27

What is the von Frey test?

Answer 27

uses increasingly stiff wires to poke foot and elicit withdrawal (mechanical force)

Question 28

What is tail pressure?

Answer 28

nociceptive mechanical force

Question 29

What is a hot plate?

Answer 29

thermal nociceptive test